1. Field of the Invention
The present invention is directed to a process of drying a cast polymeric film disposed on a workpiece. More specifically, the present invention is directed to a process of drying a cast polymeric film disposed on a workpiece by contacting the film with liquid or supercritical carbon dioxide.
2. Background of the Prior Art
The casting of polymeric films by such methods as spin coating, spraying, dip coating or roller coating is commonly employed in many commercially important processing operations. A common concern in these processes is the removal of the solvent constituent of the polymeric composition to obtain a polymeric film free of solvent.
A particularly important example of such processing involves the application of polymeric films on semiconductor workpieces in the fabrication of semiconductor devices. The most important of these polymeric films are photoresist compositions, which are disposed on workpieces by spin coating. Commonly, photoresist compositions are disposed on semiconductor wafers. This spin coating step is followed by baking, on a elevated temperature surface such as a hot plate, to drive off the solvent. This standard method of drying cast photoresists films on semiconductor workpieces is discussed in W. Moreau, xe2x80x9cSemiconductor Lithography,xe2x80x9d Chapter 7, Plenum Press, 1988. Unfortunately, hot plate drying of photoresists surfaces removes most of the solvent but, as taught by Ito et al., J. Photopolymer Sci., 1, 625 (1999), up to about 7% by weight of the solvent may remain in the cast photoresist film. This residual solvent, as taught by U.S. Pat. Nos. 5,492,793 6,043,003 and Asakawa et al., J. Vac. Sci. Technol. B, 13(3), 833-838 (May./Jun. 1995), adversely affects photoresists, especially chemically amplified photoresists. Moreover, some photoresists, as taught by Kwong et al. Spie Proc 3999, 591 (2000), are heat sensitive and decompose at baking temperatures of 100xc2x0 C. Specifically, photoresists known as xe2x80x9cfastxe2x80x9d photoresists, which contain acid amplifiers, are particularly susceptible to this problem.
An alternative method of removing solvent contained in photoresists, vacuum drying, although an improvement, does not remove all the solvent in a photoresist composition. The aforementioned Ito et al. reference indicates that up to 4% by weight of the solvent remains in the cast film after even one complete month of vacuum drying. The retention of even 4% solvent in a photoresist film, as those skilled in the art are aware, reduces lithographic resolution of the photoresist film.
Other polymeric films, which are applied in solution, especially those that are sprayed upon surfaces, such as protective coatings including enamels, varnishes, polyurethanes and lacquers, have, in the past, required the presence of organic solvents to reduce their viscosity. These organic solvents are usually volatile organic compounds (VOCs). As such, the adverse environmental impact of such materials have led to increasing governmental regulation of their use. This has resulted in attempts to reduce the use of these VOCs by the development of several alternative compositions. Thus, such alternatives as high solids coating, water-based coatings, powder coatings, non-aqueous dispersions and supercritical carbon dioxide compositions have been developed. None of these alternatives, however, provide the excellent results obtained by spray coating of protective coatings. Thus, the complete elimination of VOCs, consistent with the production of a high performance coating, has, up to the present time, not been obtained.
It is appreciated that the development of supercritical carbon dioxide compositions, as described in U.S. Pat. No. 5,106,650, represent a significant advance in the art. However, many high performance coatings employ amine/epoxy systems. Such systems are not compatible with supercritical carbon dioxide. This is so insofar as conventional amine active sites are highly reactive with carbon dioxide and tend to form amine carbamates that reversibly release carbon dioxide. This reaction causes undesirable results insofar as the resultant film is characterized by the presence of crystals, voids and the like.
It is this result that discourages spin coating of photoresist-liquid carbon dioxide solutions of the type described in U.S. Pat. No. 6,001,418. Therefore, although this positive tone development is a significant advance in the art, it is not employable when positive tone photoresists are utilized. Since most photoresist employed in the manufacture of semiconductor devices are positive photoresists, the invention of the ""418 patent is not commercializable.
U.S. Pat. No. 5,716,763 describes a method of baking a photoresist coating onto a semiconductor mask blank which overcomes problems identified in the prior art for performing this task. In this method a semiconductor substrate is immersed in a heated liquid, e.g. a silicone oil, for a time and at a temperature sufficient to obtain a uniform temperature throughout the substrate. Insofar as the heated-liquid is chemically inert with respect to the temperature sensitive photoresist, this teaching has no relationship to the drying of a polymeric film.
The above remarks establishes the need in the art for a new process of drying cast photoresist and other polymeric VOC-containing films. Such a desired process would permit drying of the photoresist or other polymeric film at temperatures far below typical baking temperatures currently employed to remove VOCs.
A new process has been developed for removing volatile organic compounds from photoresist and other polymeric compositions which are cast or sprayed onto a substrate. In this new process environmental problems associated with the removal of VOCs and, concurrently, the removal of residual amounts of these volatile compounds, which produce nonuniform coatings, is provided.
In accordance with the present invention a process is provided for drying photoresists and other polymeric coatings and films that are cast upon workpieces. In this process, the photoresist or other polymeric film, which includes a volatile organic compound therein, disposed upon a workpiece, is contacted with a liquid or supercritical carbon dioxide wherein volatile organic compounds contained therein are extracted therefrom.
In further accordance with the present invention an apparatus for drying a polymeric film composition disposed upon a workpiece is set forth. In that apparatus means for immersing a workpiece, upon which a polymeric film composition is disposed, in liquid or supercritical carbon dioxide is provided.